1. Field of the Invention
This invention relates to a flexible graphite laminate and more particularly refers to a new and improved flexible graphite laminate having reduced corrosive properties and method of producing it.
2. Description of the Prior Art
Flexible graphite laminate is a known, commercially used product, especially for packing and seals, made from graphite. The graphite is usually first treated with an oxidizing medium which is frequently an acid and an oxidizing agent. The product may be a graphite compound or graphite salt such as graphite-hydrogen sulfate. The graphite after treatment with the oxidizing medium is rapidly heated causing the volume of the graphite to materially increase producing a product termed expanded graphite. The expanded graphite is compacted or compressed to form shaped parts usually as thin sheet or thicker laminate structures, designated collectively as a flexible graphite laminate. The flexible graphite laminates have been found to be especially useful for seals and packing.
German Pat. No. 66,804 discloses that flexible graphite laminates can be produced by dipping flake-form natural graphite into a liquid oxidizing agent and subsequently heating it for a few seconds to a temperature of around 1000.degree. C. During this treatment, the individual graphite particles expand to a considerable extent and may subsequently be compacted, for example by rolling or stamping, without the addition of a special binder, to form flexible sheet-form structures, for example thin sheets or thicker laminate-like structures. The temperature-resistant self-lubricating structure which is produced is impermeable to liquids and gases and used as a seal and packing in contact with different materials, such as metals, specifically steel and ferrous alloys.
It has been found that some types of steel undergo corrosion when in contact with such graphite laminates, the degree of corrosion being determined by the nature of the surrounding medium and by the type and quantity of the impurities present in the laminate. For example, chrome-nickel steels undergo corrosion in contact with graphite laminate when in fully deionised water, but not in normal tap water. Although the degree of corrosion can be reduced by purifying the graphite laminate, it is not possible completely to prevent corrosion, even when employing graphite with very low ash contents of less than 40 ppm. Furthermore, the purification of graphite laminates, for example treatment of the laminate with halogen-containing gases at an elevated temperature, is expensive and can adversely affect the properties of the laminate, such as its flexibility.